Solder particle deposition

ABSTRACT

Solder particles  220  are deposited onto metallized contacts of a direct chip attach (DCA) site located on a substrate  301 . The contacts  302  are coated with a layer of flux  303 . A pick up head  211  is positioned in a reservoir  201  of solder particles  220  and particles are attracted to the apertures in the end  213  of the head. The apertures have an arrangement corresponding to the footprint of the metallized contacts on the substrate. The head  211  is positioned adjacent the substrate  301  and the particles  220  released. The particles  220  stick to the flux  303  coated on the contacts  302 . The particles are reflowed, leveled and again coated with flux. An integrated circuit chip is then placed on the leveled reliefs  501  and the reliefs reflowed again to attach the chip onto the contacts  302.

This is a division of Ser. No. 08/399,265 May 17, 1995 now Pat. No. 5,788,143.

FIELD OF THE INVENTION

This invention relates to attachment of integrated circuits to substrates in electronic equipment and more particularly to attachment of direct chip attach (DCA) integrated circuits (or chips) to printed circuit boards using solid solder particles.

BACKGROUND OF THE INVENTION

Presently, DCA chips are bonded to metallized contacts (DCA sites) arranged on substrates such as printed circuit boards (PCB's) by applying molten solder particles to a substrate, flattening the particles, applying sticky flux to the substrate, placing components (DCA chips) onto the flattened particles and reflowing the joints.

IBM Research Disclosure 27109 describes a tool for placing solid solder balls on a ceramic surface solder device. The balls are of the order of 0.625 mm diameter. The ceramic surface mount device is screen printed with sticky solder paste only at contact sites and the device positioned above a template having an arrangement of apertures containing balls positioned to correspond to the sticky solder paste sites. The balls are held in place on the template by gravity with assistance from a vacuum, which is released to allow the sticky solder paste on the surface mount device to extract the balls from the template. The use of a template with a vacuum also aids the installation of the balls onto the template as the balls are simply spilled over and fall into place on the template.

DCA sites have contacts at a very fine pitch, so it is critical that an exact volume of solder is deposited onto the DCA sites to form a joint. Too little solder will result in an open circuit, while too much solder will result in a short circuit. Another problem is that solder pastes cannot be readily printed to such a fine pitch, as clogging of the apertures used for printing results. In addition, paste registration to DCA sites over a whole substrate would be impossible.

A repair of an open circuit or a short circuit contact for this technology is difficult so a method that can guarantee accurate deposition volume to minimize repair activity is desired. Deposition of molten solder requires an inner and outer mask to be accurately aligned to each other, and then the combination aligned to the DCA site. The alignment of the inner and outer masks is both difficult and time consuming with no guarantee that the finished mask will give the required volumes on all DCA sites. Other variable parameters which must be precisely controlled include the thickness of inner and outer masks used to deposit the molten solder, hole size of the inner mask, stand off height of the outer mask and the air pressure which forces molten solder through the masks.

SUMMARY OF THE INVENTION

The invention provides a method of depositing solder from a reservoir onto metallized contacts arranged on a surface via a pick-up head comprising applying flux to the contacts, picking up solder in the form of particles from the reservoir with the pick-up head, the head having apertures corresponding to the arrangement of the contacts, positioning the head adjacent the contacts, aligning the apertures with the contacts, and releasing the particles onto the surface, the particles being retained on the surface by the flux.

Preferably, the method further comprises, after releasing the particles, reflowing the solder particles, leveling the reflowed particles with a flattened head to form reliefs, applying flux to the reliefs, placing integrated circuit contacts onto the reliefs, and reflowing the reliefs to attach the chip to the surface. The pick-up head preferably picks up and releases the solder particles by applying and releasing a vacuum. The solder particles are preferably maintained in a fluidized form while being held in the reservoir. This prevents the solder particles sticking together.

In a preferred embodiment of the present invention, there is provided an apparatus for denting solder from a reservoir onto metallized contacts on a surface comprising means for supplying a surface, the surface having a plurality of contacts arranged thereon, to the apparatus, a reservoir for containing solder particles, the particle size corresponding to the size of the contacts, means for producing a vacuum, a pick-up head having a connection for the vacuum producing means and a plurality of apertures smaller in size than the solder particles and having an aperture configuration corresponding to the arrangement of the plurality of contacts, the head being capable of being positioned in a first position adjacent the reservoir and a second position adjacent the surface, means for dispensing flux onto the contacts, control means for causing the head to transport the particles from the reservoir to the surface, means for aligning the apertures with the contacts and means for controlling the vacuum to the pick up head.

Preferably, the apparatus further comprises a rotating turret with the heads positioned circumferentially. The turret has an indexing system and is computer controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, with like components in different figures being referenced by like reference numerals, in which:

FIG. 1 is an overall view of an embodiment of the invention;

FIG. 2 is a cross-section view of a pick-up head and a solder reservoir used in the apparatus of FIG. 1;

FIG. 3 is a diagram of the head of FIG. 2 with solder particles held by vacuum positioned over a substrate;

FIG. 4 is a diagram of the head of FIG. 3 after release of the vacuum;

FIG. 5 is a diagram of the reflowing of the solder particles;

FIG. 6 is a diagram of another embodiment of the invention including an indexing turret.

FIG. 7 shows the circuit board assembly of the invention;

FIG. 8 shows the flattening head of the invention;

FIG. 9 shows a module of the invention;

FIG. 10 shows the acrosol flux sprayer of the invention;

FIG. 11 shows the control system of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The manufacture of solder particles of the required size (approximately 0.125 mm) to a tight tolerance (+/−0.0125 mm) is well known. The present invention uses particles of this size in place of the molten solder used in prior art solder depositing systems.

FIG. 1 shows an overall view of an embodiment of the invention, the apparatus being indicated by reference numeral 100. The apparatus comprises an indexing turret head 600 containing various heads (described later with reference to FIG. 6) including a vacuum pick up head (211 in FIG. 2), a component bank 111 containing DCA chips 115, a programmable Cartesian coordinate gantry 113 for positioning the heads contained within the indexing turret head 600 in position over a substrate 301. It also comprises two vision systems, the first 110 being used for alignment of the DCA site on the substrate 301, the second 114 being used for positioning and alignment of the pick-up bead and the DCA chip site. The substrate is positioned under the vision system 110 using a programmable “XY” table 112.

FIG. 2 shows a vacuum pick-up head 211 having at a first end 213 a template having apertures. These apertures have a footprint corresponding to the footprint of a DCA chip site on a substrate. A second end 212 of the head has a tube 214 which connects via a control means 215 to a vacuum pump (not shown). There is also a reservoir 201 of solder particles 220 in the form of a fluidized bed 202. A fluidized bed is used because the solder particles 220 being very small tend to stick together. The means for achieving a fluidized bed 202 of solder particles 220 are similar to that used for many other types of small particles and so will not be described further. The pick-up head 211 initially has no suction applied via control means 215 and tube 214 from the vacuum pump. The pick-up head 211 is lowered into the reservoir 201 and the suction from the vacuum pump applied via the control means 215 and tube 214. Air flows into the pick-up head 211 through the apertures in the first end 213 and solder particles 220 are attracted from the reservoir 201 to engage with holes in the template at the first end 213 of the head 211.

FIG. 3 shows the particles 220 held in position against the first end 213 of the head 211 by the vacuum which continues to be applied. The pick up head 211 now transports the solder spheres 220 to the substrate 301 where a machine vision system (110) is used to align the hole pattern in the template with the metallized contacts 302 on the DCA chip site. As described earlier the metallized contacts 302, together with the rest of the substrate, were sprayed with a layer of flux 303. For clarity the sticky flux has been shown only on the contacts of the DCA site. Also described earlier, these holes have a pattern corresponding to the footprint of the DCA chip site. FIG. 3 shows the pick up head 211 after the holes in the template have been aligned with the footprint of the DCA chip site. The pick-up head 211 is then positioned so that it is spaced approximately 0.075 mm from the surface of the substrate 301.

FIG. 4 shows the vacuum released so that the particles 220 fall under the influence of gravity onto contacts 302 of the DCA chip sites. The particles 220 are retained on the chip sites by the sticky flux 303 sprayed earlier onto the whole of the DCA site.

The pick-up head 211 is now removed from its position near to the DCA site and a reflow head 511, such as that shown in FIG. 5, containing a heat source 512 is positioned over the site. Heat is applied from the reflow head to the solder particles. The reflow head is moved away from the site and a flattening head (not shown) is then positioned above the DCA site and used to level the molten solder particles 501 into reliefs in the form of flattened bumps.

An aerosol fluxing head such as the one used earlier is now positioned over the DCA site and flux is again sprayed over the whole of a single DCA site from the aerosol head.

A placement head picks up a chip from a component bank and transports the chip from the component bank to a position above the DCA site. The chip contacts are then aligned with the flattened reliefs located on the footprint of the DCA site and the chip placed onto the DCA site. The chip is retained in place by the sticky flux sprayed earlier over the whole of the DCA site. A reflow head 511 such as the one used earlier is now used to reflow the flattened reliefs and attach the chip to the metallized contacts 302 on the substrate 301.

FIG. 6 shows an indexing turret head 600 used in a further embodiment of the invention. The indexing head contains an aerosol fluxing head 613, a vacuum pick up head 211, a reflow head 511, a flattening head 611 and a placement head 612.

An enhancement to the embodiments described includes the use of positive pressure applied to the pick-up head 211 to assist ejection of the solder particles 220 onto the DCA chip site contacts.

FIG. 7 shows a substrate 700 such as a direct connect chip or ball grid array chip carrier with a pattern of metal contact pads 702 which consist of a layer 704 of high melting temperature metal such as copper, aluminum, silver, or lead rich solder and a conforming layer 706 of conductive adhesive or low melting temperature metal such as eutectic Pb/Sn solder which is wetable to the high melting temperature metal. A preform 708 such as a ball of a high melting temperature metal such as Pb/Sn solder with 90-97% Pb, which is wetable by the conforming layer is placed on the contact pads. During placement, curing, or reflow the conforming layer changes shape to conform to the surfaces of the pad and preform and connects the preforms to the pad.

FIG. 8 shows a substrate 800 with a pattern of metal contact pads 802 over which a sticky flux 804 has been sprayed. Metal balls 806 such as solder or copper preforms have been positioned on each pad and are mechanically flattened by leveling head 810 as shown.

FIG. 9 shows leads 902 of a surface mount component 900 such as a direct chip or a ball grid array chip carrier module are positioned on the metal balls. During reflow the solder 806 wets the pad and lead to connect the lead to the pad after cooling.

FIG. 10 shows conduit 1000 having spray nozzles 1002 for dispensing a fine mist 1004 of flux such as a no clean flux with an alcohol or water carrier.

FIG. 11 shows a movement control system such as a robot or turret head 1100 for moving spray head 1102 and vacuum suction head 1104. A source of vacuum 1106 such as a vacuum pump and a source of positive pressure 1108 such as a compressor are connected by hoses with the suction head through valves 1110 and 1112 respectively. A vision system including a source of radiation and a focused detector such as a camera 1114 is connected 1116 with a computer controller 1118. The computer controller is connected 1120 to automatically move the robot 1100 to produce circuit boards depending on signals from the machine vision system. Computer 1118 is connected at 1122 and 1124 to control valve motors 1126 and 1128 respectively for valves 1110 and 1112 respectively.

In the invention of Applicant's it is preferred that the metal balls are agitated by the formation of a fluidized bed of particles, but alternately the balls could be agitated by applying sound, vibration, or other forces to the particles within the reservoir. 

What is claimed is:
 1. A method of making an electrical interconnect assembly, said method comprising the steps of: providing a substrate having a metal wiring layer including a pattern of high melting temperature metal contact pads as part thereof; positioning a low melting temperature conforming layer, wettable to said contact pads, on selected ones of said metal contact pads; positioning a high melting temperature metal preform above said selected ones of said metal contact pads having said conforming layer thereon, and thereafter releasing said metal preform to drop onto said conforming layer on said selected ones of said metal contact pads; and heating said conforming layer on said metal contact pad to cause said conforming layer to conform to the surface of said contact pad and to cause said metal preform to connect to said contact pad.
 2. The method of claim 1 wherein said conforming layer is provided from conductive adhesive.
 3. The method of claim 2 wherein said conductive adhesive layer is cured as a result of said heating.
 4. The method of claim 1 wherein said positioning of said metal preform above said conforming layer and said releasing and dropping of said preform includes the step of exerting a predetermined pressure onto said preform. 